Fiber Bragg gratings (FBGs) have attracted much attention since K. O. Hill et al. inscribed a FBG in a silica fiber successfully using standing-wave technique in 1978. Nowadays, FBGs have been widely used in optical fiber sensing, optical fiber communications and fiber lasers due to advantages such as small size, light weight, high sensitivity, and high resistance to electromagnetic interference. Researchers have developed many kinds of FBGs, including apodized FBGs, high-reflection FBGs, chirped FBGs, narrow-reflection-band FBGs, and phase-shifted FBGs, etc., for various applications. However, the gratings manufactured by the standing-wave technique can only operate near the inscription laser wavelength and the photosensitivity of the fiber decreases dramatically at wavelengths larger than 500 nm. As a result, the standing-wave technique has not been widely adopted. Many other manufacturing techniques are developed, including:
(1) Holographic Inscription Technique
Proposed by G Meltz et al., the holographic inscription technique can be used for manufacturing high-quality FBGs. The most prominent advantage of the holographic inscription technique is the capability of adjusting the grating period in a broad range conveniently by changing the angle between the two interference beams, so as to change the reflection wavelength. However, the gratings manufactured using this technique cannot be very long because the inscription beam spot cannot be expanded too much due to the limited power of the inscription ultraviolet laser. Moreover, it is not easy to manufacture chirped gratings or apodized gratings using this technique, and thus its application is very limited.
(2) Phase-Mask Technique
K. O. Hill et al. proposed a phase-mask technique in 1993. This technique is non-interferometric in nature, and thus greatly alleviates requirements for the coherence of ultraviolet beam and for the low-vibration manufacture environment. It also substantially lowers the difficulties in manufacturing FBGs and improves the manufacturing efficiency. Moreover, excimer lasers with higher energy and larger beam spot can be employed in this technique and thus it is suitable for mass-production. Since characteristics (e.g., length, uniformity, and period, etc.) of FBGs are mainly determined by the phase-mask, this technique can be used to manufacture chirped FBGs and phase-shifted FBGs, etc., by employing suitable masks. However, for a given phase-mask, although the grating period can be adjusted by, e.g., applying pre-stress to the fiber during the fabrication or adjusting divergence of the inscription laser beam, the adjustable range is very limited. Furthermore, the phase masks for FBGs with various reflection wavelengths are costly, and thus it is inconvenient for the manufacture of FBGs with arbitrary reflection wavelength.
(3) Point-by-Point Inscription Technique
According to this technique, the fiber grating can be manufactured by exposing a short section of the fiber to a focused single pulse and moving the fiber by a distance of the grating period before a next pulse arrives. This technique can be adaptively applied in manufacturing various kinds of fiber gratings, such as FBGs with various reflection wavelengths, chirped FBGs, and phase-shifted FBGs, etc., by precisely controlling the moving distances. However, this technique has a few limitations despite its high flexibility. First of all, it is time-consuming and thus is only suitable for manufacturing short FBGs. Second, this technique requires accurate movement control of the fiber, and the accumulation of movement errors makes it impractical for long gratings. Third, it is not an easy task to focus the laser beam to a small spot size that is only a fraction of the grating period, so it is difficult to manufacture FBGs with reflected wavelengths less than 1 μm.
In view of the foregoing techniques, the present disclosure aims to remedy the above drawbacks by providing a method and apparatus suitable for manufacturing various kinds of FBGs, such as apodized FBGs, chirped FBGs, and phase-shifted FBGs, etc., with reflection wavelengths tunable in a large range, and also suitable for manufacturing ultralong FBGs.